The present invention relates to an ultrasonic vibration unit for an ultrasonic welding device for sealing and welding material webs, comprising a sonotrode connected to a converter and having at least one sealing horn radially protruding from an axis of rotation and rotatable about the axis of rotation and having a free end forming a sealing surface.
Ultrasonic welding is a method for joining plastics. Ultrasound is a mechanical vibration above the auditory threshold. The frequency range begins at approximately 20 kHz and extends up to frequencies of 1 GHz. Such ultrasonic frequencies are frequently produced from electrical energy with the help of piezoelectric sound transducers (converters). This mechanical vibration energy is applied to the workpiece or the material being processed via the sonotrode connected to the converter, if required via an amplitude transformation unit (booster). The surface of the sonotrode, which is provided to enter into contact with the material being processed, is also referred to as a sealing surface.
The ultrasonic vibration unit therefore constitutes a structure that vibrates during operation and comprises the converter, if required the booster and the sonotrode.
In order to effectively transfer the ultrasonic vibration using the ultrasonic vibration unit, it is necessary to put the ultrasonic vibration unit into resonance. Depending on the design of the ultrasonic vibration unit, said unit has a plurality of natural frequencies. Only if the converter produces a natural frequency of the ultrasonic vibration unit does a resonant vibration of said ultrasonic vibration unit occur. For that reason, converter and ultrasonic vibration unit have to be synchronized to each other.
Strictly speaking, the resonance frequency differs somewhat from the natural frequency because each real system is attenuated. In the following—as is also frequently the case in the technical literature—the terms resonance frequency and natural frequency are used synonymously. The most important natural frequency of the ultrasonic vibration unit is generally the natural frequency whereat a standing longitudinal vibration comprising wave nodes and wave troughs develops in the ultrasonic vibration unit. In doing so, a wave trough develops in each case at the face ends of the sonotrode.
The converter is connected to one of the face ends, which produces the corresponding ultrasonic excitation frequency. If applicable, an amplitude transformer (booster) is connected between converter and sonotrode, which booster changes the amplitude of the ultrasonic vibration but not the frequency. By providing a booster, the natural frequency of the sonotrode and therefore the position of the node of the longitudinal vibration are not influenced.
In many of the applications, the amplitude transformation unit is integrally formed with the sonotrode, i.e. can no longer be optically differentiated. In order therefore to differentiate the sonotrode from said amplitude transformation unit, it is necessary to determine the position of the vibration antinodes of the pure longitudinal vibration. The sonotrode generally comprises the sealing surface. Each section, which extends in the longitudinal direction from vibration maximum to vibration maximum and which does not influence the natural frequency of the pure longitudinal vibration, is not part of the sonotrode. If, on the other hand, such a section influences the natural frequency of the pure longitudinal vibration, i.e. said section cannot be removed without a significant change in the natural frequency occurring, said section then belongs to the sonotrode.
When processing materials with the aid of ultrasound, the material to be processed is generally positioned between the sonotrode and a counter tool, which does not belong to the vibration structure and is also referred to as the anvil. The sonotrode which is in contact with the material to be processed transfers the ultrasonic energy to the material to be processed, which is thereby, for example, welded or detached. The heat required to plasticize the material web is produced by converting ultrasonic vibrations into frictional energy. Due to the interfacial and molecular friction, heat thus develops which causes the plastic to start to melt.
In the case of most sonotrodes, the longitudinal ultrasonic vibration is used for transferring energy over the sealing surface.
There are however also sonotrodes having a sealing surface substantially in the shape of a cylinder jacket surface. These sonotrodes use the radial ultrasonic vibration, which forms transversely to the longitudinal direction of propagation of the ultrasonic vibration, for transferring energy. Said sonotrodes frequently consist of a substantially rod-shaped section, to which the converter and if applicable the booster are connected, and a wheel-shaped or bell-shaped section protruding radially over the rod-shaped section. The wheel- or bell-shaped section comprises the sealing surface.
These sonotrodes have generally two main natural vibration modes.
The one natural vibration mode corresponds substantially to the longitudinal resonance vibration of the rod-shaped section. Said resonance vibration has a relatively large longitudinal vibration amplitude. This is however associated with a forced influencing of the material in the transverse direction, i.e. perpendicularly to the rod axis. Said forced influencing manifests itself in a thickness vibration, which propagates radially with respect to the rod axis. The vibration amplitude of the thickness vibration is relatively small, which results in the majority (more than 90%) of the vibration energy absorbed in the vibration system being contained in the longitudinal vibration.
The other natural vibration mode corresponds substantially to the resonance of the radial vibration of the wheel-shaped section. A comparatively small (forced) vibration in the longitudinal direction is associated therewith. In said natural vibration mode, the majority (usually more than 90%) of the vibration energy absorbed in the vibration system is contained in the radial vibration.
During rotation welding, the second natural vibration mode is used because by generating a relatively small longitudinal vibration in the rod-shaped section of the sonotrode, a relatively large radial vibration in the wheel-shaped section of the sonotrode can be generated.
Sonotrodes are thus known which have a sealing surface in the shape of a cylindrical surface and are used for the continuous ultrasonic processing of moved material webs. Said sonotrodes are rotated about the longitudinal axis thereof during operation so that the cylinder surface-shaped sealing surface moves substantially at the same speed as the material web to be processed. In the case of said sonotrodes, only a small part of the sealing surface is therefore in contact with the material web.
An ultrasonic welding device of the kind mentioned at the beginning of the present application is known from the WIPO patent application WO 2007/012917 A1. The device of multi-rotor design consists of two rotating shafts disposed parallel to one another. Sonotrodes are on the one shaft and anvils are mounted o the other shaft. The converters serving to feed the sonotrodes are likewise situated on the rotating shaft. A disadvantage with said device is that a converter is required for each sonotrode. In the case of, e.g., a four rotor embodiment, four converters are therefore necessary, which leads to high costs. Converters and sonotrodes have a relatively large overall height, and therefore the center distance of the two parallel shafts has to be selected large. This large center distance requires much space and has a negative effect on the format size range. In the case of a format change from, e.g., a 3-rotor design to a 4-rotor design, a complicated conversion entailing a difficult subsequent alignment is required. This results in greater downtimes and longer startup times after the change in format.
The WIPO patent application WO 2009/156207 A1 discloses a rotating sonotrode, which has a roller-like effective area (sealing surface). The sonotrode is of wavelike design and is optionally furnished with boosters. The sealing surfaces are embodied as radial raised sections that are wheel-shaped or bell-shaped. The sonotrode is supplied axially with ultrasonic energy by a converter. The sonotrode, which is of wavelike design, is embodied as a unit and must therefore in accordance with the vibration properties thereof be designed as a total system. If so-called rotors instead of a roller-like sealing surface are then to be embodied in a sonotrode for a tubular bag machine, this configuration has also to be designed as a total system. In addition, constraints arise with regard to the number of rotors. Hence, the system has, e.g., to be of symmetrical construction, which only allows for an even number of rotors. A replacement of the sonotrode due to maintenance work or also in the case of initial installation is only possible as a whole and is thereby costly. Even changing the number of rotors due to a format adjustment requires a removal of the entire sonotrode which entails a complicated subsequent alignment.
The WIPO patent application WO 02/060674 A1 discloses a rotating sonotrode which is axially fed on both sides with ultrasonic energy. Due to the conical shape and a corresponding hollow space in the sonotrode, the vibration is redirected about an angle of 90° in the radial direction. The advantage of said sonotrode is the possibility of a two-sided mounting, whereby a higher sealing pressure can be generated. The distribution of the vibration on the sealing surface can also be more evenly executed in comparison to conventional conversion of the axial vibration energy into radial vibration energy by means of transverse contraction. Due to the hollow space, only cylindrical shapes are possible for the sonotrode. Sonotrodes having a plurality of rotors, as is required for a tubular bag machine, cannot be implemented on account of the thin, conical walls required for redirecting the waves, said walls reducing the stability of the sonotrode. A further disadvantage is the reduced sealing width and reduced energy distribution on the effective surface. Thus, a limited format range arises as well as an insufficient sealing quality for wide sealing seams.